I always take the o-rings off and use Permatex RTV sealant on them instead...A little UV additive and a 4" UV cold cathode is a great way to look for leaks..How could you not see a bunch of little leaks everywhere?

Man I'll leave you my cleaning and set-up PDF, it has a lot of the basics in it like that....

the leaks aint dripping atall...they litterally tiny little leaks that evaporate....only way to see theirs water by leaving a tissue their for like an hour and theirs a single drop.....soo its leaking VERY VERY little...but enufff to make my water level drop!

Oh OK...Those are the little weepy kind, I hate those..Yeah use some UV like everyone was saying, plus you can run a finger around the unions to and see if your finger gets a little wet...Those kind will really shine under a UV light if you have some UV additive...

I don't know what kind they have over there, but here I use "Tracer-Line" UV additive, most auto stores have it and it only takes a couple of drops in your loop...Its 5 or 6 bux for an ounce here, I'm still using a bottle that I got a year or two ago...1 ounce is good for a cars whole cooling system, its concentrated stuff...